Driving myself crazy with lung cancer fear age 33 smoker, gp says lungs are clear, slight back/shoulder pain & an on/off cough for months gp says sinus?

Cancer phobia: If your had bone pain from metastatic cancer, this would show up on your x-rays. You also realize that you're young to have lung cancer, and that your cough is very likely due to your smoking unless your physician has found another cause such as a tendency to asthma. This would be a great time to stop smoking because the next scare may be real & get with your physician about dealing with fear.

Not serious: If you were really worried, you would quit smoking for real. Then you can get a ct scan of the chest. Beyond that, there is not much your doctor can do.